Aircraft window heating system with regulated dc voltage

ABSTRACT

The example non-limiting technology herein provides an alternative way to control window heater temperature by delivering to the heater a DC voltage with variable level.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD

The technology herein relates to electronic controllers used for windshield or window heating systems e.g., in aircraft.

BACKGROUND & SUMMARY

Window anti-icing, deicing, and defogging systems are used to keep windshield areas free of ice, frost, and fog. These can be electric, pneumatic, or chemical depending on the type and complexity of the aircraft.

High performance and transport category aircraft windshields are typically made of laminated glass, polycarbonate, or similar ply material. Typically clear vinyl plies are also included to improve performance characteristics. The laminations create the strength and impact resistance of the windshield assembly. The laminated construction facilitates the inclusion of electric heating elements into the glass layers, which are used to keep the windshield clear of ice, frost, and fog. The elements can be in the form of resistance wires or a transparent conductive material may be used as one of the window plies. To ensure enough heating is applied to the outside of the windshield, heating elements are often placed on the inside of the outer glass ply.

Whether resistance wires or a laminated conductive film is used, aircraft window heat systems often have transformers to supply power and feedback mechanisms, such as thermistors, to provide a window heat control unit with information used to keep operating temperature within acceptable limits. Some systems are automatic while others are controlled by cockpit switches. Separate circuits for pilot and co-pilot are common to ensure visibility in case of a malfunction. See Ice and Rain Protection, https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch15.pdf

While much work has been done in the past, further improvements are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of exemplary non-limiting illustrative embodiments is to be read in conjunction with the drawings of which:

FIG. 1A shows an example non-limiting aircraft windshield.

FIG. 1B is a cross-section of the example FIG. 1A windshield.

FIG. 1C is a block diagram of an example window heating system.

FIG. 2 is a block diagram of an example two-channel window heating controller.

FIG. 3 is a more detailed block circuit diagram of the example window heating controller.

FIG. 4A is a signal diagram showing incoming DC voltage.

FIG. 4B is a signal diagram showing the output of switching circuit 306.

FIG. 4C is a signal diagram showing the output of output filter 314.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1A shows an aircraft 100 including a heated windshield 102. Current aircraft electrical window or windshield heating systems are commonly designed to operate in accordance with the mains electrical generation type: alternating current (AC) or direct current (DC). Larger aircraft generally use AC electrical systems whereas smaller aircraft generally use DC electrical systems. The window heater of a DC system aircraft generally uses a filament heater type, due to low resistance needed for power dissipation on the windshield.

In case of an AC system, the window heater element is often made of a thin and uniform film embedded within the transparent material. See FIG. 1B. The controller unit for an AC system aircraft basically is designed to deliver a sinusoidal signal with a phase shift. This phase shift is controlled according to the heater temperature (which may be monitored for example by a temperature sensor such as a thermistor), so the heater temperature will be almost constant during operation.

The heater controller unit for a DC system aircraft often provides two different types of control: ON-OFF, and variable Pulse Width Modulation:

1) ON-OFF: the controller is designed to keep the heater temperature within two temperature limits, there being a significant temperature difference between the two temperature limits. However, this temperature difference is enough to cause, under some flight conditions, an optical diffraction in the windshield, due to the temperature gradient made by external temperature and heater temperature. This effect occurs until the window temperature stabilizes, but it may disturb the pilot's vision for a few seconds.

2) Pulse width modulation (PWM): the controller delivers to the heater a switched power, with the signal width (i.e., the width of a period power signal) being controlled according to the heater temperature. Using PWM, the heater temperature tends to be constant. A drawback of this solution is that often, depending on the size of the windshield and the heat output of the heater, a high power (large current) needs to be switched. Periodically switching a high power potentially generates excessive electromagnetic interference to other aircraft systems.

The example non-limiting technology herein provides an alternative way to control window heater temperature by delivering to the heater a DC voltage with variable level.

FIG. 1C shows a proposed heating system comprising a DC electrical supply (1), window heating resistive elements 112 with embedded temperature sensing elements (2) 202, an electronic controller 200 to control the power delivered to the heating elements based on current heater temperature (3), and, power contactors connected between DC power supply and electronic controller (4), commanded by the controller whenever is appropriate.

The heater controller is composed by: an input LC type filter, a high power and high frequency switch, an output LC filter, analog signals reading circuit, a Control Law circuit for error calculation and control of a high frequency PWM signal, and, a failure monitor circuit for misbehavior detect of each system component.

The input voltage to the controller must be within 20-30 VDC range (5), so the output voltage will be between 0V and the input voltage value less the internal drop voltage. This output is possible through a circuit based on a high frequency PWM signal switching a MOSFET gate (8), getting a 28V level switched right after the said MOSFET (6). Then, the switched voltage is transformed to the mean DC level (7) by the output filter (9). In addition, the input voltage pass through an input filter (10) also before being switched by MOSFETs, to guarantee there is no harmonics being conducted to the electrical supply bus.

The duty cycle of the PWM signal may vary from 0% to 100%, this percentage will be controlled by Control Law circuit (12). The output from Control Law circuit is a 0-1V signal that controls a PWM circuit (11), whom will generate a PWM signal with a duty cycle proportional to this input signal. The Control Law circuit receives the temperature reading signal and calculates the error compared to a reference temperature (38° C.). The temperature reading is made by a proper analog conditioning circuit (14), giving a signal proportional to the temperature sensor resistance. However, this temperature signal is monitored by a dedicated circuit in order to detect if it is still in valid range or unwanted value (overheat) (13). Also, this circuit monitors the output current and input voltage, in order to avoid operating in failure conditions.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A window heat controller system comprising: an Electrical System supplying 28 VDC in one or more independent buses; a controller capable to receive 28 VDC and to output a DC voltage regulated between 0 and 28 VDC for two independent heating elements, output which is regulated according to the heating element's temperature, including over-current protection and over-heat monitor on heating element through its temperature sensing element; and, power contactors for controlled power supply shut-off made by said controller; and, an electrical resistance heating element connected to receive controlled power from said controller with an embedded temperature sensing means for developing a DC signal proportional to the temperature of said heating element.
 2. An electrical resistance heater controller, as said in claim 1, for controlling application of power from a DC source to a heating element with fixed resistance value and temperature sensing elements embedded, comprising: switching DC power input means in a fixed frequency being gating by a PWM signal with variable duty cycle; input filter means included before the said switching element for attenuating harmonics generated by this element being returned to power input buses; output filter means connected after said switching element for changing the switched input voltage to a proportional DC voltage level according to the duty cycle of said switched input voltage, and also for attenuating the harmonics generated by said switching element being delivered to the said electrical resistance heating element; and, sensing, controlling and protection means using the said embedded temperature sensing, said output filter delivered current and said input filter voltage level, for keeping the heating element temperature constant, protected against over-heat and over-current.
 3. An electrical resistance heater controller in accordance with claim 2 further comprising: an analog signal conditioning means receiving the temperature sensing signals, output current and input voltage level and converting them to an appropriate DC signal level; a failure monitoring circuit capable to detect the misbehavior of the controller or other system components with proper indication for each kind of failure through output signals and proper command of said power contactors; a control law means monitoring the current temperature level of heating element and acting to correct this temperature to a pre-determined level, keeping this temperature as constant as possible; a PWM signal circuit means to generate a PWM signal with duty cycle proportional to the control signal indicated by said control law. 